In the exploration for oil and gas, it is necessary to drill a wellbore into the Earth. While drilling of the wellbore permits individuals and companies to evaluate sub-surface materials and to extract desired hydrocarbons, many problems are encountered.
For example, it is well known that the “easy oil” is generally gone. Exploration now requires searching to greater depths than ever before. This necessitates drilling deeper and deeper, and thus into harsh environments, such as those having temperatures ranging from 200° C. up to or in excess of 300° C. Generally, present day instrumentation is not built to operate in such an environment, and will fail well before reaching ambient temperatures within this range.
The growing complexity of downhole instrumentation further complicates this problem. That is, as technology continues to improve, exploration is making use of more instrumentation than ever before. With this usage comes an increased demand for power downhole. In addition, downhole instruments becoming available have greater instantaneous (or pulse or peak) power requirements. For example, certain downhole instruments may be able to use an existing downhole power source while operating in a first mode, e.g., a standby mode, but require a high power pulse, which existing power sources cannot readily meet, during a second mode of operation, e.g., a data collection or transmission mode.
Unfortunately, many of the known downhole power sources have substantial drawbacks. For example, various types of batteries suffer catastrophic failure at elevated temperature, and can thus destroy instrumentation. Meeting the high instantaneous (or peak or pulse) power demand of certain downhole instruments requires high rate batteries, which typically have a lower capacity than low or medium rate batteries and are more susceptible to catastrophic failure at elevated temperatures. Additionally, batteries currently used in downhole applications are typically not rechargeable and may be quite expensive. When a battery requires replacement, e.g., due to failure or charge depletion, a drilling operation must be halted while the drillstring, typically thousands of linear feet, is extracted from the well to gain access to the batteries and any instrumentation that may also require replacement. This operation is time consuming and expensive, and potentially hazardous.
Another currently available downhole power source is a downhole generator, e.g., a turbine-based generator. Downhole generators may not suffer from the same temperature limitations as available downhole battery technologies, but downhole electrical generators are highly complex and expensive devices. For example, a typical high temperature, high pressure downhole turbine generator is designed to withstand temperatures up to 200° C. to 300° C., pressures in the thousands of pounds per square inch (psi), shock and vibrational forces up to several hundred g, and exposure to corrosive chemicals present in the drilling mud. Thus, downhole generators are typically constructed out of expensive, highly engineered materials, similar to those found in expensive jet engines or other gas turbines. In terms of electrical performance, downhole generators also suffer from many of the same limitations as batteries, being unable to provide consistently high power pulses to meet the requirements of many downhole instruments.
Electromagnetic (“EM”) telemetry tools are an example of a class of downhole instruments with complex power requirements, particularly pulse power requirements. EM telemetry involves transmitting information about subsurface conditions to the surface using an EM signal, as opposed to mud pulse (“MP”) telemetry where information is transmitted by mechanically varying the pressure of the drilling fluid (or mud) in the wellbore. EM telemetry typically has a higher data transfer (or bit) rate (about 10 bits per second (“bps”)) than MP telemetry (about 1-4 bps). In addition, MP telemetry is not suited to many complex drilling operations, e.g., directional drilling or underbalanced drilling, where EM telemetry is necessary. Therefore, EM telemetry is necessary or favored during many drilling operations. The strength of an EM telemetry signal is directly related to the power—a stronger, more powerful EM telemetry signal can propagate over a longer distance and/or have a higher bit rate. For example, many conventional EM telemetry tools cannot operate at depths greater than a few thousand feet because signal attenuation renders the signal undetectable at the surface receiver. In addition, many conventional EM telemetry tools have efficiency limitations that prevent them from delivering a high power EM signal.
Therefore, a high power EM telemetry device is needed that is capable of efficiently providing high power EM telemetry signals in a downhole environment, where temperatures range from ambient environmental temperatures up to about 200° C. Celsius or higher, including up to about 300° C.